In present-day fuel metering systems, efforts are made to adhere to the most precise possible stoichiometric fuel-air mixture because of both fuel consumption and exhaust emissions. However, the stoichiometric mixture has not proved to be efficacious over all the operating ranges of an engine. An example is full-load operation, in which the primary importance is to produce the maximum possible engine torque. If the fuel metering system includes a lambda regulating device having a lambda sensor in the exhaust tube, then the regulation system is generally shut off during full-load operation and a switchover is made to open-loop control of fuel metering. "Full-load operation" is, as a rule, defined by a throttle valve which is entirely, or virtually entirely, opened.
Depending upon the behavior of the driver of a vehicle equipped with such a fuel metering system for the engine, full-load operation may be signalled very frequently and the fuel metering system switched over from closed-loop to open-loop control. Over a relatively long period the intermittent enrichment of the mixture during the full-load phases results in unfavorable exhaust emissions.